Powder metallurgy (P/M) is a well-established process for the fabrication of near net shape components. In press and sinter applications for example, the powder is compacted in a die to form a green compact. The compact is then ejected from the die and sintered to create metallurgical bonds between the particles. A lubricant is generally required to improve the compressibility of metal powders and also to reduce the powder/die wall friction in order to facilitate ejection and minimize die wear.
Die wall lubrication technology as disclosed in U.S. Pat. No. 3,871,877 to Storchheim allows the fabrication of compacts having high green strength and sintered properties. However, the use of die wall lubrication generally lowers the production rate and the technology is not always applicable for some part geometry (high die fill for example). Accordingly, for some applications, the use of admixed lubricants is the only alternative for the production of sound parts at low cost.
Lubricants are commonly admixed to metal powders for the compaction of powders by P/M techniques. However, known admixed lubricants have deleterious effects on some properties of the compacts. For example, admixed lubricants generally reduce interparticular cold welding and reduce the green strength of the compacts. This may be troublesome if the green strength is not sufficient to prevent delamination or crack formation during shaping or handling. This is particularly the case when spherical aluminum powders are used. In addition, admixed lubricant may negatively affect the sintered properties of the compacts. By reducing the interparticular friction, the lubricant may impede the formation of good metallurgical bonds between the particles during compaction and may thus affect the sintered properties of the parts. In addition, the reduction of the sintered properties may be important if the lubricant leaves residual products that impede the formation of metallurgical bonds during sintering. In fact, for aluminum P/M applications, the lubricant should burn out cleanly at a low temperature (generally not higher than 430.degree. C.) to avoid the reaction of aluminum with the lubricant or residual products during sintering. Since aluminum sintering is sensible to the presence of oxygen, the lubricant should also be burned out in inert atmosphere.
Unlike for other metal powders, the only popular admixed lubricants for the fabrication of aluminum P/M components are synthetic amide waxes such as ethylene bisstearamide (EBS). EBS is a good lubricant that burns out cleanly at a low temperature. However, in comparison with parts fabricated with die wall lubrication, aluminum parts fabricated with admixed EBS have lower mechanical properties.
U.S. Pat. No 5,498,276 to Luk discloses powder compositions containing a poly(ethylene oxide) lubricant for the fabrication of compacts with improved green strength. However, this lubricant leaves carbon residues when heated in inert atmosphere at temperatures lower than 1000.degree. C. For that reason, this lubricant should not be used for the fabrication of aluminum P/M components.
Polyethylene has been used as a lubricant for the fabrication of certain P/M components having improved green strength. It has also been used as a lubricant for the fabrication of iron components for press-and-sinter applications. However, decomposition of PE is reported to be complete at temperatures higher than 520.degree. C. (J. N. Auborn and J. S. Choo, "Effect of Chemistry and Compact Density on the Decomposition of P/M Lubricants", Advances in Powder Metallurgy and Particulate Materials, 1994, (3), 103-116). Most ceramic and metal P/M components are sintered at temperatures much higher than the decomposition temperature of this lubricant. However, unlike for other metal or ceramic powders, this temperature limitation poses a problem for the fabrication of aluminum P/M components since Al parts must be free from lubricant residuals after a delubrication at a temperature lower than 430.degree. C., according to H. C. Newbing and G. Jangg, "Sintering of Aluminum Parts: The State-of-the-Art", Metal Powder Report, May 1987, (42), 354-358.